U.S. Pat. No. 5,549,561 and U.S. Pat. No. 5,435,076 show injection cartridges of the dual-chamber type comprising a barrel with a distal end and a proximal end. In the barrel, there is provided a proximal piston which is displaceable within the barrel. A further displaceable piston is provided in the distal direction of the proximal piston to define a first chamber between the two pistons. The distal end of the barrel is closed by a rubber septum which is held in place with a metal crimp.
The needles for use with such a cartridge usually consist of a needle with an attached hub. The needle hub fits around the outside of the cartridge closure and carries features to attach the needle to the cartridge or to an injection device, for example a screw thread. Such needles are typically supplied as a separate component to the injection device and are attached manually by the user prior to injection. The needles are supplied with a plastic outer cover which can be used to provide a means for handling the needle and for protecting the user from potential needle stick injuries.
The injection cartridge of U.S. Pat. No. 5,549,561 or U.S. Pat. No. 5,435,076 has a distal end which is configured as a neck with a reduced inner diameter. This neck allows fastening of the metal crimp. Needles typically used for such a cartridge have a distal tip and a proximal tip, the latter penetrating the rubber septum when attaching the needle to the cartridge. Thus, the proximal tip of the needle protrudes into the neck of the injection cartridge. This typically leaves a dead space in the neck of the cartridge. This is a volume of a cartridge from which the contents cannot be accessed where a quantity of drug product can remain after administration.
The dead space creates a number of disadvantages. Additional medicament may need to be filled to compensate for dead space losses. This is generally not desired and is a commercial disadvantage, especially for expensive products. Also, in applications where the entire content of the cartridge is dispensed any variability of the dead space volume will have a direct impact on the accuracy of the delivered dose.
The problem of dead space is particularly relevant for delivery of suspensions because it may cause the suspension to become inhomogeneously mixed. For example the dead space known from U.S. Pat. No. 5,549,561 or U.S. Pat. No. 5,435,076 may preferentially trap solid particles (or microspheres) because it is also an area of relatively slow or stationary fluid flow.
There are two reasons why this dead space is present. Firstly, the design of the device, needle and cartridge closure must ensure that the needle will always pierce the septum, irrespective of manufacturing tolerances. Secondly, the design of the device, needle and piston in the cartridge must ensure that the piston never hits the needle tip.
Current needles are designed such that the needle protrudes some way past the septum and into the cartridge as shown in U.S. Pat. No. 5,435,076 or GB 705,392. This is to take account of various manufacturing and assembly tolerances that could affect whether or not the needle pierces the septum. For example, needle length, septum thickness, screw thread tolerances on needle hub and medical device, crimping strength, glass cartridge dimensions, degree to which the user tightens the needle on the screw thread etc.
GB 705,392 discloses a syringe having at its distal end a sleeve-like stopper with a rubber septum integrally formed in this stopper. A needle may be attached to this syringe such that the proximal tip of the needle pierces the rubber septum. Again, the stopper defines a dead space for receiving the proximal tip of the needle.
EP 0 261 318 A1 describes an injection device with a cartridge which has a distal end and which is closed by a rubber member. Within the distal end of the cartridge, a needle is provided which may be forwarded to pierce the rubber element and to protrude from the distal end of the cartridge as a piston is forwarded within the cartridge. This configuration of the cartridge and the needle again creates a relatively large dead space in the distal end of the cartridge.
Further, WO 84/02079 A1 refers to a cartridge having at its distal end a cartridge closure in the form of a cap enclosing the distal end of the cartridge. A central distal portion of this cap is configured as a hollow tip protruding in the distal direction of the cartridge. This hollow tip is closed by a septum which is integrally formed with the cap and the tip. Opening of this known cartridge may be achieved by pushing a short needle with a single, proximal tip in the protruding tip of the cap thus piercing the septum so that the needle tip protrudes into the distal end of the cartridge. As the distal end of the cartridge is formed with a neck with a reduced inner diameter, again a dead space is created.
In addition to the drawbacks of a dead space, known cartridges face the risk of bending of the needle piercing the septum outside the center and piercing into side-walls of the closure inside the cartridge which may damage or block the needle. This risk of an improper alignment of the needle with respect to the septum is caused by the fact that the needle usually is not guided with respect to the septum.